weather vane.

a blade, plate, sail, etc., in the wheel of a windmill, to be moved by the air.

any of a number of blades or plates attached radially to a rotating drum or cylinder, as in a turbine or pump, that move or are moved by a fluid, as steam, water, hot gases, or air.

a person who is readily changeable or fickle.

Aerospace .

any fixed or movable plane surface on the outside of a rocket providing directional control while the rocket is within the atmosphere.

a similar plane surface located in the exhaust jet of a reaction engine, providing directional control while the engine is firing.

Ornithology . the web of a feather.

Navigation, Surveying . either of two fixed projections for sighting an alidade or the like.

Archery . feather ( def. 5 ) .

noun

fan , feather , weathercock